Intervertebral prostheses are used for replacing the intervertebral disk. They comprise two cover plates, whose outer surfaces are designed for connection to adjacent vertebral bodies, and an articulation device enclosed by the cover plates. In a known prosthesis (EP-B 471 821), the upper cover plate forms a concavely spherical articulation surface on its inner side, which cooperates with the convexly spherical top surface of a prosthesis core of polyethylene in order to form an articulation. The core has a flat underside and a cylindrical edge which are received with matching fit in a seat which is formed on the inside by the lower cover plate.
In order to permit easy flexion movement, it would be useful to choose a small radius of curvature of the articulation surfaces. However, this would lead to a small corresponding surface area of the articulation surfaces and would lessen the capacity for load transmission. In said known prosthesis, the radius of curvature of the articulation surface is chosen approximately equal to half the prosthesis diameter. This has the disadvantage that the prosthesis has a considerable structural height and in many cases cannot be accommodated in the restricted intervertebral space. If, in said type of prosthesis, one were to choose a still greater articulation radius, so that the core assumed the shape of a flat sphere section, it would be found that the articulation properties of the prosthesis leave something to be desired.